Structure and superconductivity in compressed Li-Si-H compounds: Density functional theory calculations

Previous experimental evidence showed that silane ($\mathrm{Si}{\mathrm{H}}_{4}$) becomes a superconductive phase at a critical temperature (${T}_{\mathrm{c}}$) of 17 K above 96 GPa, although this observation was not supported by later experiments due to the fact that $\mathrm{Si}{\mathrm{H}}_{4}$ was measured to decompose into amorphous silicon and solid hydrogen at $\ensuremath{\sim}60--90\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and then recrystallized into a nonmetallic phase up to $\ensuremath{\sim}130\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Given lithium has a short atomic radius and low electronegativity, it could be incorporated into the binary hydrides and act as an electron donor by doping electrons into the lattice of parent binary hydrides, enabling the modification of crystal structures and superconductivity for the resulting ternary hydride system. In this work, therefore, we attempted to chemically tune crystal structures and improve the superconductivity of the Si-H system via lithium incorporation, by performing structure searching simulations on the Li-Si-H system at a wide pressure range of 50--350 GPa. As a result, four stable stoichiometries of $\mathrm{LiSi}{\mathrm{H}}_{5}, \mathrm{LiSi}{\mathrm{H}}_{6}, \mathrm{LiS}{\mathrm{i}}_{2}{\mathrm{H}}_{9}$, and $\mathrm{L}{\mathrm{i}}_{2}\mathrm{Si}{\mathrm{H}}_{6}$, as well as two metastable stoichiometries of $\mathrm{LiSi}{\mathrm{H}}_{4}$ and $\mathrm{LiSi}{\mathrm{H}}_{8}$, were uncovered under high pressures. Among these predicted stoichiometries, $\mathrm{LiS}{\mathrm{i}}_{2}{\mathrm{H}}_{9}$ and $\mathrm{LiSi}{\mathrm{H}}_{8}$ are predicted to become good phonon-mediated superconductors with estimated ${T}_{\mathrm{c}}$ of 54 and 77 K at 172 and 250 GPa, respectively. These results highlight the role of lithium incorporation in chemically tuning crystal structures, which induce the significant change of electronic properties for a Si-H system from nonmetallicity or poor metallicity to high-${T}_{\mathrm{c}}$ superconductivity, shedding light on the exploration and discovery of high-${T}_{\mathrm{c}}$ superconductivity in a variety of ternary hydrogen-rich compounds associated with lithium incorporation.